low E
f
values may not be adequate for high-temperature applications, they
may be perfectly adequate for the low temperatures encountered in underwater
cables.
14.8.2 Amplifier Fibers
Carbon coatings have been used to protect erbium-doped amplifier fibers.
These fibers play a critical role in commercial telecommunications and in spe-
cialty applications. In their nonhermetic version, these fibers have the same
bending limitations as other fibers. This limits the physical size of amplifier
modules because the bend radii used in winding and routing the fiber cannot
be less than a critical value, typically about 33 mm. By using hermetic carbon
coating, the problem of fatigue is eliminated, allowing tighter fiber coils and
more compact amplifier modules.